This invention relates to sintered polycrystalline compacts of cubic boron nitride for use in machining tools, abrasives, wire dies, wear parts, heat sinks. and the like. More particularly, this invention relates to a processes for preparing a sintered polycrystalline compact of cubic boron nitride, and the compacts produced thereby.
The high pressure forms of boron nitride, known as cubic boron nitride and wurzitic boron nitride, are surpassed only by diamond in hardness and wear resistance. Wurzitic boron nitride, typically formed by shock or explosive techniques, has a hardness equal to the cubic form and can be substituted or mixed with the cubic form. The Wurzitic form of boron nitride is thermodynamically unstable relative to the cubic form under conditions favorable to sintering and will therefore revert to the cubic form in the presence of catalyst/solvents. For convenience, the abbreviation "CBN" is intended by the inventors in the following disclosure and claims to refer to both the cubic and the wurzitic high pressure form of boron nitride.
In similar fashion as diamond, CBN has proven particularly useful when made into a bonded polycrystalline mass, often referred to as a "sintered CBN compact" or simply a "CBN compact". In particular, CBN compacts are preferred to diamond compacts in certain applications, such as working with ferrous metals, because CBN is chemically more stable than diamond.
Although it is possible to form a sintered CBN compact with no binder material under conditions of high pressure and temperature, strongly adherent surface oxides of boron inhibit intergranular bonding and make it difficult, if not impossible, to obtain adequate compact strength. Various binder materials are thus incorporated, either to enhance intercrystalline bonding or to surround the grains with a continuous adhering and/or supporting matrix. It is this first type of binder, i.e. one that enhances intercrystalline or "CBN to CBN" bonding and is sometimes called a catalyst/binder, with which the present invention is concerned. In particular, the invention is an improvement in binder systems which use elemental silicon (including alloys of silicon) to promote this intercrystalline bonding. This is to be distinguished from the various processes which use silicon and its compounds to produce the matrix type binder system for a CBN compact. For example, see U.S. Pat. No. 4,353,953 to Morelock and U.S. Pat. No. 4,110,084 to Lee which both show the use of a silicon carbide containing matrix for supporting crystals of CBN.
Silicon has several properties which have made it a desirable constituent in a catalyst/binder system for forming polycrystalline CBN compacts with substantial intercrystalline or CBN to CBN bonding. First, silicon is essentially non-reactive with boron nitride. Second boron nitride is virtually insoluble in silicon. Third, silicon does form a variety of alloys and intermetallic compounds with many substances which do react with boron nitride. For example, aluminum, the lanthanides, transition metals such as molybdenum, tungsten, titanium, zirconium, and halfnium all form such alloys and intermetallic compounds with silicon. As a result of these first three properties, silicon can be used in combination with these materials which do react with CBN as a diluent to limit or regulate the extent of interaction with the CBN. Silicon is thus used to enable controlled dissolution and recystallization of CBN, thereby enhancing the formation of intergranular bonding in the compact.
A fifth property which makes silicon a desirable ingredient in catalyst/binder systems for CBN compacts is its ability to promote wetting and bonding between the CBN and a wide variety of refractory hard-metal and ceramic materials which can also be present in the binder. A sixth property is that, in contrast to most other materials, the liquid phase of silicon is more dense than the solid phase. As a result, the application of high pressures substantially reduces the melting point of silicon thereby making it more active as a solvent at lower temperatures.
An example of the benefits derived from using elemental silicon in a catalyst/binder system for a polycrystalline CBN compact is taught in the inventors' co-pending U.S. patent application Ser. No. 666,459. In this application, the inventors teach a binder system comprising elemental silicon together with an aluminum containing material. The compact produced possesses substantial intercrystalline bonding.
However, one problem which the present inventors did note in the compacts produced by the above-described process, was an excessive amount of cracking of the compacts during cooling at normal rates. In particular, a high percentage of these compacts had to be rejected due to this cracking of the polycrystalline structure. The inventors deduced that the problem was due, at least in part, to the presence of the elemental silicon in the compact. That is, as to the compact cooled, the silicon reverted to the solid phase and expanded within the pores of the polycrystalline network. Therefore, although the peculiar property of silicon being less dense in the solid phase proved beneficial at the time of the formation of the compact, this same property was also creating a serious problem after the compact was formed and was cooling.
A partial solution to this problem was achieved by providing extended periods for cooling each compact at a high pressure maintained in the press. This method helped maintain the polycrystalline structure intact as the solidifying silicon was forced to extrude to accommodate for its expansion. However, this method proved unsatisfactory in that it reduced the efficiency of compact production in terms of press utilization and manpower requirements. In addition, even with very slow cooling, (up to ten minutes total), about half of the compacts still showed cracks. Also, it is the inventors' belief that in all cases the remaining silicon induced at least some residual strain in the polycrystalline structure thus weakening the compacts.
Another problem with using elemental silicon in the catalyst/binder system of a polycrystalline CBN compact is the fact that elemental silicon is a relatively soft material. In particular, because elemental silicon is not particularly hard or wear-resistant, the residual elemental silicon detracts from the hardness or wear resistance of the compact.
Another problem inherent in most polycrystalline CBN compacts, including the compact produced according to the inventors'co-pending application Ser. No. 666,459, is the fact that they cannot be cut with a conventional Electric Discharge Machine (EDM). The EDM, which uses an electrical spark to cut, has proven quite effective in cutting precision shapes in ultra-wear-resistant and otherwise difficult to cut materials such as polycrystalline diamond with a metallic binder and cemented tungsten carbide. Unfortunately however, this same method has not been successful with polycrystalline CBN. For example, neither the metal-backed CBN compact marketed by the General Electric Company under the trade name "BZN" (believed by the inventors to be produced by the process disclosed in U.S. Pat. No. 3,743,489), nor the indexable CBN compact marketed by DeBeers under the trade name "Amborite" (believed by the inventors to be produced by the process disclosed in U.S. Pat. No. 3,944,398) can be cut with an EDM. As a result, required shapes for polycrystalline CBN tool inserts and the like must be obtained by either molding the end shape in the press, grinding, or other difficult and expensive techniques.
It is interesting to note that the "BZN" compact has a metallic binder phase and is electrically conducting. The CBN compact of the inventor's co-pending application Ser. No. 666,459 is likewise electrically conducting. As a result, one would expect that these two compacts would behave like polycrystalline diamond compacts with metallic binders which are EDM cuttable. As to the reason for the difference, it is the inventors' understanding that although a spark can be initiated through the metallic binder phase, the CBN adjacent to the spark reverts either to hexagonal boron nitride ("HBN", the low pressure form) or to a B.sub.2 O.sub.3 glass, both of which are non-conducting and could coat and thereby insulate the adjacent metallic binder phase, thus quenching the spark. In contrast, the diamond in the polycrystalline diamond compact reverts to graphite (the low pressure form) which is conducting and therefore maintains the spark.
There is one metal-backed CBN compact, which is marketed by Sumitomo under the trade name "Sumiboron" and believed by the inventors to be made by one of the processes disclosed in U.S. Pat. No. 4,343,651 or U.S. Pat. No. 4,334,928, that can be cut by EDM. The relatively large volume fraction (up to 40% of the total volume of the compact) of metallic titanium carbide or titanium nitride contained in this compact's binder material is believed to impart this property. However, this high volume fraction of binder material also inhibits intercrystalline CBN bonding and reduces the abrasion resistance of the compact.